Kissing, it's a romantic action that many of us have probably engaged in. But why do human beings feel compelled to place their lips on someone else’s lips and exchange pressure, saliva, and 80 million different types of bacteria? The answer is a little bit of science and a little bit of culture.

Human beings are complicated creatures and deciphering why we do the things we do is no small feat: The act of kissing is no different. Academics and scientists have puzzled over the intricacies of how two mouths crashing into one another came to symbolize love, sexual desire, reverence, and even a simple greeting. Is it nature? Yes. Is it nurture? Yes. It’s all of the above.

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Nature's role in why we kiss.

When it comes to human behavior there’s a lot to be learned from animals. You may have recognized that leaning in for a smooch with your cat or dog more often than not elicits the opposite reaction from them. The truth is, most creatures do not kiss. Sure there are a few animals who partake in lip moves—chimpanzees and bonobo, for example, exhibit kissing behavior, though not necessarily in a romantic context—but in general human beings are the only creatures that kiss in a romantic sense. So why do we do it?

Scientists say that it’s a simple case of pheromones. A pheromone is a chemical that an animal or insect produces that affects the behavior of another animal in the same species. In this instance, pheromones work to inform the opposite sex of a viable mate. For example, male wild boars produce a pungent smell (pheromone) that lady boars find attractive. Lady hamsters emit a pheromone that Mr. hamsters find enticing, and female black widows produce a scent that informs males when it’s safe to mate. All of this olfactory information is registered by the opposite sex without the animal or insect having to get particularly close to one another. And while human beings have a lot going for them, a great sense of smell is not one of those things. So we get close. Really close.

“In many cultures, kissing was one of the first opportunities for individuals to get close enough to sniff each other in socially acceptable ways,” Rafael Wlodarski, an Oxford University doctoral candidate and lead researcher of a study on the function of kissing, told the New York Times. The study was published in the Archives of Sexual Behavior, and it infers that kissing essentially evolved from that sniffing behavior turning into a romantic, physical liplock. “It's hard to pinpoint when this happened,” Wlodarski told the BBC earlier this year, “but both serve the same purpose.”

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Kissing is also a learned behavior.

Which brings us to this: kissing is as much a learned behavior as it is a biological one. Some anthropologists speculate that kissing evolved from “kiss feeding,” the process by which mothers pass masticated food from their mouths to their babies mouths. While some historians think that romantic kissing is a product of “western culture, being passed on from one generation to the next,” It may even be a little bit of both.

The oldest evidence of kissing can be found in Hindu Vedic Sanskrit texts from 1500 B.C. describing a custom of rubbing and pressing noses together. "Eventually, someone slipped and found that the lips were very sensitive and found it pleasurable,” Vaughn Bryant, a Texas A&M University anthropologist and ardent kissing historian, told Discovery News. "That's one theory on how it started.”

From there Eastern culture influences Greek culture where we find kissing referenced in Homer’s epics and in Herodotus’ "The Histories." Romans followed this cultural influence. They were such ardent kissers that they had three different words for distinguishing a kiss on the hand (osculum), a kiss on the lips (basium), and a deep passionate smooch (savolium). In the Middle Ages, a kiss was a signal of social standing and even a legal bind, and during the Industrial Revolution, a kiss to the hand became the predecessor of today’s handshake. It’s easy to see the cause and effect that advancing globalization had on the custom of kissing.

Most hunter-gatherer societies, on the other hand didn’t—and don’t—kiss at all. Recent research from the University of Nevada published in American Anthropologist, surveyed 168 cultures and found that only 46 percent of them participate in romantic kissing. The others? Not so much, though some cultures participate in other romantic physical gestures. The Inuit gently press their nostrils to the cheeks or forehead of loved ones to inhale their scent. And others abstain all together. Anthropologists studying equatorial and sub-Saharan hunter-gatherer societies note a “marked absence of kissing.” There are even cultures that find kissing “revolting,” with one indigenous Brazilian culture, the Mehinaku, asking an ethnographer why anyone would want to "share their dinner.”

Joe Hanson's "It's OK to be Smart" explains the science of kissing in a PBS Digital Studios video below: